While epoxy-based composites are suitable for many applications, their brittle nature and susceptibility to degradation make them inadequate for many aerospace applications, especially those applications which require thermally stable, tough composites. Accordingly, research has recently focused upon polyimide composites to achieve an acceptable balance between thermal stability, solvent resistance, and toughness. Still the maximum temperatures for use of the polyimide composites, such as PMR-15, are about 600-625.degree. F., since they have glass transition temperatures of about 690 .degree. F.
Polybenzoxazoles, such as those disclosed in copending U.S. application Ser. No. 651,862, may be used at temperatures up to about 750-775.degree. F., since these composites have glass transition temperatures of about 840.degree. F. Aerospace applications need composites which have even higher use temperatures while maintaining toughness, solvent resistance, processibility, formability, strength, and impact resistance.
Chemists have sought to synthesize these new oligomers for high performance advanced composites, resulting in a progression of compounds synthesized to provide unique properties or combinations of properties. For example, Kwiatkowski and Brode disclosed maleic capped linear polyarylimides in U.S. Pat. No. 3,839,287. Holub and Evans disclosed maleic or nadic capped imido-substituted polyester compositions in U.S. Pat. No. 3,729,446. Lubowitz and Sheppard disclosed thermally stable polysulfone oligomers in U.S. Pat. No. 4,476,184, and have continued to make advances with polyetherimidesulfones, polybenzoxzolesulfones, and polybutadienesulfones. "Star" and "star-burst" multidimensional oligomers exhibit surprisingly high glass transition temperatures, as described in copending U.S. application Ser. No. 726,258. The multidimensional oligomers have superior processibility than some advanced composite oligomers since they can be handled at lower temperatures. Upon curing, however, the thermal resistance of the resulting composite is markedly increased with only a minor loss of stiffness, matrix stress transfer (impact resistance), toughness, elasticity, and other mechanical properties.
While polyesters are among the most highly developed polymers, commercial polyesters do not exhibit satisfactory thermal and oxidative resistance to be useful for aircraft or aerospace applications. Even polyarylesters are unsatisfactory, since the resins are generally insoluble in laminating solvents, are intractable in fusion, and shrink or warp during composite fabrication. The high concentration of ester groups contributes to resin strength and tenacity, but also makes the resin susceptable to the damaging effects of water absorption. High moisture absorption can lead to distortion of the composite when it is loaded at elevated temperature.
Conductive and semiconductive plastics have been extensively studied (see, e.g., U.S. Pat. Nos. 4,375,427; 4,338,222; 3,966,987; 4,344,869; and 4,344,870). These prior art polymers do not possess the blend of properties which are essential for aerospace applications. That is, the conductive polymers do not possess the blend of (1) toughness (2) stiffness, (3) elasticity, (4) processibility, (5) impact resistance and other matrix stress transfer capabilities, (6) retention of properties over a broad range of temperatures, and (7) high temperature resistance which is desirable on aerospace advanced composites. The prior art composites are often too brittle.
As described in copending U.S. application Ser. No. 726,259, high performance, aerospace advanced composites can be prepared using crosslinkable, end capped polyester imide ether sulfone oligomers that have the desired combined properties of solvent resistance, toughness, impact resistance, strength, processibility, formability, and thermal resistance. By including Schiff base (-CH.dbd.N-), imidazole, thiazole, or oxazole linkages in the oligomer chain, the linear, advanced composites formed with oligomers of U.S. application Ser. No. 726,259 can have semiconductive or conductive properties when appropriately doped.
As described in copending U.S. application Ser. No. 726,258, glass transition temperatures above 900.degree. F. are achievable with multidimensional, branched oligomers which include crosslinking groups at the ends of linear aromatic "sulfone" arms radiating from an aromatic hub. When cured to crosslink the end caps, these "star" and "star-burst" oligomers provide a multidimensional array. The resulting thermally stable composite exhibits excellent toughness and processibility. The crosslinking groups (usually nadic and acetylenic phenylimide moieties) also provide solvent resistance to the composites. Glass transition temperatures of about 950.degree. F. or higher are achievable.
The present invention combines features of the "Schiff base" conductive sulfone polyarylesters with the "star" and "star-burst" multidimensional morphology to create advanced conductive composites.